Automatic control of oxygen removal from process streams



R- A. TlEDE May 20, 1958 AUTOMATIC CONTROL OF OXYGEN REMOVAL FROMPROCESS STREAMS Filed Oct. 18, 154

2 Sheets-Sheet 2 A T TORNEYS United States Patent AUTOMATIC CONTROL OFOXYGEN REMOVAL FROM PROCESS STREAMS Raymond A. Tiede, Burger, Tex.,assignor to Phillips Petroleum Company, a corporation of DelawareApplication October 18, 1954, Serial No. 462,790

17 Claims. (Cl. 260-677) This invention relates to automatic control ofoxygen removal from process streams. vention relates to a method andapparatus for measuring the oxygen absorbing capacity of a liquid. In amore specific aspect, this invention relates to the automatic control ofan oxygen-removing unit in a process for handling a polymerizablematerial, such as an olefin.

It is known in the art that oxygen exerts deleterious elie'cts upon manyprocesses. Polymerization processes, in particular, are known to beadversely affected by the presence of oxygen in the polymerizablematerials. Other reactions, such as hydrogenation, alkylation andisomerization are also known to be adversely afiected by the presence ofoxygen in the process materials.

Thus, oxygen is an undesirable constituent of process materials in manydifferent chemical processes. These materials include liquid and gaseoushydrocarbons, such as gasoline and naphthas, paratiins and particularlyolefins such as ethylene, propylene and 1,3-butadiene. alytic reactionsinvolving these materials, oxygen is actually a poison for somecatalysts.

Various materials are known to be oxygen scavengers or absorbing agentsand have been used to remove oxygen from these process materials.

For example, sodium dithionite has been recommended as an oxygenscavenger for redox emulsion polymerization reactions. Other oxygenabsorbing agents are resorcinol, pyrogallol, phloroglucinol,chlorohydroquinone, quinone and hydroquinone. Recently it has been foundthat alkaline solutions of alkali-metal hyposulfites or oftertiary-butylcatechol are excellent oxygen absorbing agents, especiallyfor removing oxygen from olefin streams, such as ethylene and butadiene.

In the copending application of Thomas I. Kennedy and Raymond A. Tiedeentitled Removal of Oxygen From Hydrocarbon Streams, Serial No. 397,772,filed December 11, 1953, the use of an alkaline aqueous solution of analkali-metal hyposulfite, such as sodium hyposulfite dissolved in sodiumhydroxide, to remove oxygen from various hydrocarbon streams isdisclosed and claimed. Also, in the copending application of Sig C.Fauske entitled Removal of Oxygen From Streams Containing Diolefins andthe Like, Serial No. 331,543, filed January 16, 1953, the use of anaqueous sodium hydroxide solution containing a high concentration of thesodium salt of tertiary-butylcatechol to remove oxygen from olefinstreams is disclosed and claimed.

In the processes described in the above cited patent applications and inany process for removing oxygen from a process stream, additionalquantities of the oxygen absorbing agent must be added continuously orintermittently to the oxygen-containing stream in order to maintain apreselected low concentration of oxygen in the process stream. Suchaddition of the oxygen absorbing agent is customarily based uponperiodical laboratory analyses of the treated or untreated processstream, for example, by an Orsat analysis. Periodic analysis andperiodic control of the addition of the oxygen absorbing agent issubject to the defects inherent in such non-continuous control of aprocess, such as a non-uniform oxy- In one aspect, this in-' In cat- .t

Patented May 20, 1958 "ice ratus for continuously determining thestrength of an oxygen absorbing agent, and in response to the continuousdetermination, automatically controlling the addition of fresh oxygenabsorbing agent to the process stream.

It is a still further object to provide a method and apparatus for thecontinuous measurement of the strength of an oxygen absorbing agent andin response thereto for continuously controlling the strength of theoxygen absorbing agent used to remove'oxygen from an olefin,

such as ethylene or 1,3-butadiene.

I have discovered a system for accomplishing the foregoing objects whichis based on the discovery that the strength of an oxygen absorbing agentis proportional to the amount of oxygen which the oxygen absorbing agentcan absorb. In accordance with this invention, the oxygen-absorbingagent is supplied to a reaction chamber at a constant rate of flow. Astream of oxygen is admitted to the reaction chamber at a constantpressure and I have found that the quantity of oxygen which flows intothe reaction chamber under these conditions is the quantity of oxygenwhich reacts with the oxygen absorbing agent and is proportional to thestrength of the oxygen absorbing agent. Also, in accordance with thisinvention, an apparatus is provided for carrying out the above-describedreaction and for automatically controlling the addition of freshoxygen-absorbing agent to a process stream in order to maintain apreselected low concentration of oxygen in the process stream.

The copolymerization of butadiene and styrene is a process which showshow the present invention can be used to good advantage. It has beenobserved in this polymerization reaction that the presence of oxygen inthe butadiene causes the reaction to proceed erratically and, in somecases, can cause the polymerization to stop before the desired degree ofconversion has been obtained. Tertiary-butylcatechol is conventionallyused to prevent polymerization of the butadiene during storage and/orshipment to the copolymerization process. The abovecited applicationSerial No. 331,543 discloses that the undesired oxygen in the butadienecan be removed by increasing the content of tertiary butylcatechol inthe caustic scrubber used in treating the butadiene. It is theredisclosed that the oxygen concentration in the scrubbed butadiene shouldbe maintained between 0 and 5 parts per million and that it can be somaintained if the concentration of tertiary-butylcatechol in the causticscrubber is maintained at at least parts per million. An essentialfeature of this process is therefore the maintenance of a highconcentration of tertiary-butylcatechol in the caustic scrubber. It isespecially desired that this concentration be high enough to reduce theconcentration of oxygen in the treated butadiene to below 5 parts permillion.

In order to determine how much tertiary-butylcatechol must be added tothe caustic scrubber in order to maintain the desired low concentrationof oxygen, laboratory analysis is made of the treated or untreatedbutadiene to determine the oxygen content and the amount oftertiary-butylcatechol which is added to the caustic scrubber is basedupon this laboratory analysis. Such a procedure permits surges of oxygento go through the system without being removed and, also, graduallyincreasing amounts of oxygen may not be detected until several hoursafter the increase has occurred. 1 The method and means of thisinvention, as applied to such a commercial process, overcomes thesedisadvantages and will be better understood when considered inconjunction with the accompanying drawings wherein:

Figure 1 is a series of graphs based on data in Example I, hereinafterdescribed, which show that the strength of an oxygen absorbing. agent isproportional to the quantity of oxygen which the agent absorbs and tothe heat of reaction when the oxygen absorbing agent is reacted withoxygen; and

Figure 2 is a schematic flow diagram illustrating the use of thisinvention to control the automatic removal of oxygen from a processstream.

EXAMPLE I The oxygen absorbing agent, an alkaline solution of sodiumhyposulfite or tertiary-butylcatechol, was admitted to the top of anabsorption column at a constant, known rate of flow and a constanttemperature. The absorption column used was a glass column, inchdiameter by 3 feet, 6 inches long which was packed with inch glasshelices. tached to the bottom of the column.

Oxygen was admitted to the column near the bottom at a constant, knownpressure. A pressure regulator on the oxygen inlet line provided aconstant pressure of 20 millimeters of water and a flowmeter on theoxygen inlet line indicated the quantity of oxygen passing into theabsorption column. A temperature indicating device was also attached tothe bottom of the absorption column to indicate the heat of reaction asreflected by the temperature rise taking place due to the reaction ofthe oxygen absorbing agent with oxygen in the column.

Runs 1 through 5 in Table I, below, show the quantity of oxygen absorbedper unit time when a constant flow, milliliters per minute, of a causticsolution of sodium hyposulfite was passed through the absorption column.Each of these runs was conducted for the same length of time.

Runs 6 through 8 show the heat of reaction which was observed when theconstant pressure of oxygen was maintained on the absorption column andthe flow rate of the sodium hyposulfite solution was maintained at aconstant value.

Runs 9 through 11 show the quantity of oxygen absorbed per unit timewhen a constant flow, milliliters per minute, of a caustic solution oftertiary-butylcatechol was passed through the absorption column. Each ofthese runs was conducted for the same length of time.

Runs 12 through 14 show the heat of reaction which was observed when theconstant pressure of oxygen was maintained on the absorption column andthe flow rate of the tertiary-butylcatechol solution was maintained at aconstant value.

Table I Concentration Quantity of Heat of or Oxygen Oxygen Reaction RunOxygen Absorbing Agent Absorbing Absorbed, (Temper- No. Agent, ml. perature Weight Unit Time Rise, F.) Percent Sodium hydrosulfite- -do Aliquid trap and drain was at- 4 The data reported above in Table I showthat the strength of the oxygen absorbing agent is proportional to thequantity of oxygcnabsorbed and to the heat of reaction in the oxygenabsorbing column. These data are plotted in Figure 1, line A-Arepresenting the data for runs 1 through 5, line BB representing thedata for runs 6 through 8, line CC representing the data for runs 9through 11, and line DD representing the data for runs 12 through 14.

EXAMPLE II Runs were made which demonstrate the effectiveness of otheroxygen absorbing agents, described hereinbefore, to remove oxygen from astream of ethylene. A stream of ethylene was passed upwardly at 400 p.s. i. g. through a reaction zone, 2 inches in diameter by 5 feet longwhich contained a caustic solution of tertiary-butylcatechol orpyrogallol. The temperature of the reaction zone was maintained at F.for run No. 1 and F. for run Referring now to Figure 2, the utilizationof my invention to automatically control the removal of oxygen from aprocess stream is shown. The process which is used for illustrating myinvention in Figure 2 is the preparation of butadiene for subsequentpolymerization reactions, such as the copolymerization of butadiene withstyrene.

Fresh butadiene in line 1 and recycled butadiene from thecopolymerization reaction in line 3' is fed to blending tanks A and Bvia lines 5 and 7. From tanks A and B the blend, suitably prepared, ispassed by means of lines 9 and/or 11 and line 13 into a caustic scrubber15. A caustic solution is admitted to the scrubber 15 through line 17and tertiary-butylcatechol is admitted to the scrubber 15 through line19' and line 13. The caustic removes tertiary-butylcatechol from thebutadiene and, by maintaining a sufliciently high concentration oftertiary-butylcatechol in the caustic scrubber, oxygen is also removedfrom the butadicne by the alkali metal salt of tertiary-butylcatechol'.The combined streams of butadiene, caustic and tertiary-butylcatecholpass from the scrubber 15' through line 21 to a separator vessel 23. Thecaustic solution containing tertiary-butylcatec'hol separates from thebutadiene i-n separator 23 in a lower phase and the upper phasecontaining the treated butadiene is passed through line 25 to thecopolymerization process. The treated butadiene in line 25 can bewaterwashed by means not shown to remove any entrained caustictherefrom. The caustic solution containing tertiary-butylcatechol in thelower phase in separator 23 is recycled through line 27, line 19'andline 13 to the caustic scrubber 15. As has been explained hereinbefore,

fresh tertiary-butylcatechol must be continuously or intermittentlyadded to the recycled caustic solution in line 27 in order that asufiiciently high concentration of tertiary-butylcatechol is maintainedin the scrubber 15. This addition of fresh tertiarybutylcatechol isaccom plished automatically, in accordance with this invention, as willnow be described.

A sample stream of the partially spent caustic solution containingtertiary-butylcatechol in line 27 is withdrawn and passed through a line29 to a packed absorption column 31. A rate of flow controller 33,comprising an orifice 35 and a motor 'valve- 37 connected in line 29,supplies the sample stream in line 29 to the absorption column 31 at aconstant rate of flow. The sample stream leaves the absorption columnthrough an outlet at the bottom, a liquid trap 39 and through a line 41to a drain or other suitable use. Oxygen or other oxygen-containing gas,such as air, is admitted to the absorption column 31 at a point near theoutlet of the sample stream through a line 43. A pressure controller 45connected to line 43 actuates a motor valve 47 in line 43 and therebyprovides a stream of oxygen to the absorption column 31 at a constantpressure.

In accordance with this invention, a motor valve 49 in line 19 regulatesthe flow of makeup tertiary-butylcatechol to the caustic scrubber andmotor valve 49 is automatically controlled by the reaction proceeding inthe absorption column 31. This 'is accomplished by one of two means. Arate of flow controller 51, comprising an orifice 53 in line 43,monitors the rate of flow of oxygen in line 43 and, in response thereto,controls motor valve 49 in line 19. Alternatively, a tem-,

perature recorder controller 55 is connected to the sample outlet inabsorption column 31 and monitors the heat of reaction occurring incolumn 31 due to the reaction of the sample stream with oxygen. Inresponse to the heat of reaction so monitored, temperature recordercontroller 55 actuates motor valve 49 in line 19 so as to admit theproper amount of fresh tertiary-butylcatechol to the scrubber 15.

In the case Where an alkaline aqueous solution of an alkali-metalhyposulfide is used as the oxygen absorbing agent, such as sodiumhyposulfite, the granular sodium hyposulfite is fed from a hopper havinga vibratory feeder into a mix tank wherein the alkaline solution of thehyposulfite is prepared. In accordance with this invention, thevibratory feeder is controlled by either rate of flow controller 51 ortemperature-recorder-controller 55 and the addition of make-uphyposulfite is automatically controlled in accordance with the strengthof the oxygen absorbing agent, as described above.

It will be apparent from the foregoing description and the accompanyingdrawings that my invention broadly comprises a system for measuring theoxygen absorbing capacity of a fluid stream, comprising the oxygenreaction chamber having a suitable packing material therein, means forpassing a sample stream through the reaction chamber at a constant rateof flow, means for admitting a stream of oxygen to the reaction chamberat a constant pressure and means for measuring the quantity of oxygenwhich is admitted to the reaction chamber under these conditions. Thedata reported in Example I clearly show that the quantity of oxygenadmitted to the reaction chamber is proportional to the strength of theoxygen absorbing solution. Thus, my invention further comprises a methodand means for automatically introducing needed amounts of the oxygenabsorbing ,agent into the process stream in response to the quantity ofoxygen which is absorbed in the oxygen absorption column.

It will be further apparent from the foregoing description that myinvention broadly comprises a continuous process wherein a material (A)is contacted with a process material (B) to react therewith and therebyremove an ingredient (C) in said process material B to thereby producean ingredient C-free process material B(D) containing reacted andunreacted material A(E), wherein said material E is thereafter removedfrom said material D and returned to said process material B for reusein reacting with and removing ingredient C from said process material B,and wherein fresh, nnreacted material A(F) is continuously added to saidmaterial E to maintain a preselected minimum concentration of saidmaterial Fin said material E, a

6 method foncontinuously determining the concentrationof unreactedmaterial A material E and, in accordance therewith, for continuouslycontrolling the addition of material F to said material E to maintainsaid preselected minimum concentration of said material 'F in saidmaterial E, which comprises, continuously passing a sample (G) of saidmaterial E through a reaction zone, continuously passing a variable butdeterminable amount of material (H) comprising ingredient C through saidreaction zone to completely react sample G with ingredient C in materialH, continuously detecting the amount of ingredient C in material Hrequired for said reaction of ingredient C in material H with materialE, and continuously controlling the addition of fresh, unreactedmaterial F to material E in accordance with the amount of ingredient Cin material H required for the last-mentioned reaction.

Reasonable variation and modification are possible Within the scope ofthe foregoing disclosure and the appended claims to this invention; Itis again pointed out that my invention is applicable, broadly, to theremoval of oxygen from any fluid stream and that important applicationsof the invention are in processes for the removal of oxygen fromethylene or l,3-butadiene.

I claim:

1. An apparatus for measuring the oxygen absorbing capacity of a fluidstream which comprises an oxygen reaction chamber having a sample inletin one end and a sample outlet in the other end, means for passing saidfluid stream through said inlet, reaction chamber and outlet at aconstant rate of flow, means for admitting a stream of oxygen at aconstant pressure into said reaction chamber at a point near said outletto react.

with said fluid stream therein, and means for measuring the quantity ofoxygen admitted into said oxygen reaction chamber.

2. An apparatus for measuring the oxygen absorbing capacity of a fluidstream which comprises an elongated oxygen reaction chamber having asample inlet in one.

end and a sample outlet in the other end, liquid-vapor contacting meansin said reaction chamber, means for passing said fluid stream throughsaid inlet, reaction chamber and outlet at a constant rate of flow,means for admitting a stream of oxygen at a constant pressurecorder-controller for measuring the heat of reaction of said fluidstream with oxygen.

5. An apparatus for automatically controlling the addition of anoxygen-absorbing agent to a fluid stream to maintain a preselectedminimum concentration of said oxygen-absorbing agent in said fluidstream, which comprises, an elongated, vertical oxygen reaction chamberhaving a sample inlet near the top end and a sample outlet ,near thebottom end, a first conduit means for conveying a sample of said fluidstream into said oxygen reaction chamber inlet, a first rate of flowcontrolling means in said first conduit means for admitting said sampleto said oxygen reaction chamber at a constant rate of flow, a secondconduit means connected at one end to a source of oxygen and at theother end to said reaction chamber at a point near said sample outletfor conveying oxygen into said reaction chamber, a pressure controllingmeans in said second conduit means for admitting oxygen to the reactionchamber at a constant pressure, means for measuring the quantity ofoxygen admitted to said reaction chamber through said second conduit,and means responsive to said oxygen measuring means for automaticallycontrolling said addition of said oxygen absorbing agent to said fluidstream to maintain said preselected minimum concentration of saidoxygenabsorbing agent in said fluid stream.

6. An apparatus in accordance with claim wherein said means formeasuring the quantity of oxygen admitted to said reaction chambercomprises a rate of flow controller installed in said second conduit formeasuring the rate of flow of oxygen into said reaction chamber.

7. An apparatus in accordance with claim 5 wherein said means formeasuring the quantity of oxygen admitted to said reaction chambercomprises a temperature recorder-controller connected to said sampleoutlet for measuring the heat of reaction of oxygen with said fluidstream sample.

8. A method for automatically controlling the addition of freshoxygen-absorbing material to an at least partially spentoxygen-absorbing material which comprises, contacting a sample of the atleast partially spent oxygenabsorbing material with oxygen underconditions such that said sample is completely reacted with oxygen,measuring the amount of oxygen required for said complete reaction, andin accordance therewith, automatically controlling the addition of freshoxygen absorbing material to the at least partially spentoxygen-absorbing material.

9. In a continuous process wherein a material (A) is contacted with aprocess material (B) to react therewith and thereby remove an ingredient(C) in said process material B to thereby produce an ingredient C-freeprocess material. E(D) containing reacted and unreacted material A(E),wherein said material E is thereafter removed from said material D andreturned to said process material B for reuse in reacting with andremoving ingredient C from said process material B, and wherein fresh,unreacted material A(F) is continuously added to said material E tomaintain a preselected minimum concentration of said material F in saidmaterial E, a method for continuously determining the concentration ofunreacted material A in material E and, in accordance therewith, forcontinuously controlling the addition of material F to said material Eto maintain said preselected minimum concentration of said material F insaid material E, which comprises, continuously passing a sample (G) ofsaid material E through a reaction zone, continuously passing a variablebut determinable amount of material (H) comprising ingredient C throughsaid reaction zone to completely react sample G with ingredient C inmaterial H, continuously detecting the amount of ingredient C inmaterial H required for said reaction of ingredient C in material H withmaterial E, and continuously controlling the addition of fresh,unreacted material F to material E in accordance with the amount ofingredient C in material H required for the last-mentioned reaction.

10. In a continuous process for removing oxygen from a fluid streamwherein said fluid stream is contacted in a contacting zone with anoxygen-absorbing material and wherein said oxygen-absorbing material isthereafter separated from said fluid stream and recycled to saidcontacting Zone, a method for continuously adding make-upoxygen-absorbing material to said recycled oxygen-absorbing material tomaintain a preselected concentration of fresh oxygen-absorbing materialin said contacting zone, which comprises, passing a sample of saidrecycled oxygen-absorbing material at a constant rate of flow through anoxygen reaction zone, maintaining a constant pressure of anoxygen-containing gas on said oxygen reaction zone to thereby completelyreact said sample with oxygen, measuring the amount of oxygen reactingin said oxygen reaction zone, and in accordance therewith controllingthe addition of make-up oxygen-absorbing material to said recycledoxygen-absorbing material.

11. In a continuous process for removing oxygen from a fiuid streamwherein said fluid stream is contacted in a contacting zone with anoxygen-absorbing material and wherein said oxygen-absorbing material isthereafter sepa rated from said fluid stream and recycled to saidcontacting zone, a method for continuously adding make-upoxygen-absorbing material to said recycled oxygen-absorbing material tomaintain a preselected concentration of fresh oxygen-absorbing materialin said contacting zone, which comprises, passing a sample of saidrecycled oxygen-absorbing material at a constant rate of flow through anoxygen reaction zone, maintaining a constant pressure of anoxygen-containing gas on said oxygen reaction zone to thereby completelyreact said sample with oxygen, measuring the heat of reaction for thereaction of said sample with oxygen in the oxygen reaction zone andcontrolling the addition of make-up oxygen-absorbing material to saidrecycled oxygen-absorbing material in accordance with said heat ofreaction in said oxygen reaction zone so measured.

12. In a method for removing oxygen from an olefin stream by contactingsaid stream in a contacting zone with an oxygen-absorbing agentconsisting essentially of an alkaline aqueous solution of analkali-metal hyposulfite, wherein said oxygen-absorbing agent isthereafter separated from said olefin stream and recycled to saidcontacting zone, a method for continuously adding make-up alkali-metalhyposulfite to said recycled oxygen-absorbing agent to maintain apreselected concentration of said alkali-metal hyposulfite in saidcontacting zone, which comprises, passing a sample of said recycledoxygen-absorbing agent at a constant rate of flow through an oxygenreaction zone, maintaining a constant pressure of oxygen on said oxygenreaction zone to thereby completely react said sample with oxygen,measuring the amount of oxygen which reacts with said sample, and, inaccordance therewith, controlling the addition of make-up alkali-metalhyposulfite to said recycled oxygen-absorbing agent.

13. A method in accordance with claim 12 wherein the amount of oxygenwhich reacts in said oxygen reaction zone is measured by measuring therate of flow of said oxygen into said oxygen reaction zone.

14. A method in accordance with claim l2'wherein the amount of oxygenwhich reacts in said oxygen reaction zone is measured by measuring theheat of reaction for the reaction of oxygen with said sample of therecycled oxygen-absorbing agent in said oxygen reaction zone.

15. A method in accordance with claim 12 wherein said olefin stream isethylene.

16. A method in accordance with claim 12 wherein said olefin stream is1,3-butadiene.

17. In a method for removing oxygen from an olefin stream by contactingsaid stream in a contacting zone with an oxygen-absorbing agentconsisting essentially of an alkali metal hydroxide solution containingan alkali metal salt of tertiary-butylcatechol, wherein saidoxygen-absorbing agent is thereafter separated from said olefin streamand recycled to said contacting zone, a method for continuously addingmake-up tertiary-butylcatechol to said contacting zone to maintain apreselected concentration of said salt of tertiary-butylcatechol in saidcontacting zone,

(References on foiiowing page) 9 r 10 References Cited in the file ofthis patent 2,402,113 Hatch et a1. June 11, 1946 2,565,354 Cohen Aug.21, 1951 UNITED STATES PATENTS 2,631,175 Crouch Mar. 10, 1953 1,669,944McMichael May 15, 1928 2,047,985 Weir July 21, 1936 5 OTHER REFERENCES2,095,473 Keunecke Oct. 12, 1937 Ch d M t E M 4 2,118,567 Milaset a1 May24, 1938 114 22, 1 m ay 1 Pages 2,186,688 Walker Jan. 9, 1940 h d M t EM 194 2,222,870 Kniskern Nov. 26, 1940 128 an ngmeermg ay Pages 12,250,468 Cockerille July 29, 1941 10

1. AN APPARATUS FOR MEASURING THE OXYGEN ABSORBING CAPACITY OF A FLUIDSTREAM WHICH C OMPRISES AN OXYGEN REACTION CHAMBER HAVING A SAMPLE INLETIN ONE END AND A SAMPLE OUTLET IN THE OTHER END, MEANS FOR PASSING SAIDFLUID STEAM THROUGH SAID INLET, REACTION CHAMBER AND OUTLET AT ACONSTANT RATE OF FLOW, MEANS FOR ADMITTING A STREAM OF OXYGEN AT ACONSTANT PRESSURE INTO SAID REACTION CHAMBER AT A POINT NEAR SAID OUTLETTO REACT WITH SAID FLUID STREAM THEREIN, AND MEANS FOR MEASURING THEQUANTITY OF OXYGEN ADMITTED INTO SAID OXYGEN REACTION CHAMBER.
 8. AMETHOD FOR AUTOMATICALLY CONTROLLING THE ADDITION OF FRESHOXYGEN-ABSORBING MATERIAL TO AN AT LEAST PARTIALLY SPENTOXYGEN-ABSORBING MATERIAL WHICH COMPRISES, CONTACTING A SAMPLE OF THE ATLEAST PARTIALLY SPENT OXYGEN ABSORBING MATERIAL WITH OXYGEN UNDERCONDITIONS SUCH THAT SAID SAMPLE IS COMPLETELY REACTED WITH OXYGENMEASURING THE AMOUNT OF OXYGEN REQUIRED FOR SAID COMPLETE REACTION, ANDFIN ACCORDANCE THEREWITH, AUTOMATICALLY CONTROLLING THE ADDITION OF FRESHOXYGEN ABSORBING MATERIAL TO THE AT LEAST PARTIALLY SPENTOXYGEN-ABSORBING MATERIAL.